When such a body is subjected to a given pressure, anisotropic stresses are established which generate a birefringence effect by photoelasticity. The birefringence of an incident light ray (running longitudinally through the body) increases with pressure.
An article published in the periodical Optic Letters, Vol. 11, No. 5, May 1986, entitled "Side-hole fiber for fiber-optic pressure sensing" by M. H. Xie, Ph. Dabkiewicz, R. Ulrick, and K. Okamoto, describes a sensor used for measuring pressure and constituted by a glass optical fiber provided with longitudinal internal ducts. The variation in the birefringence of an incident light ray is measured, which birefringence increases with increasing pressure Pi inside the ducts. In a variant, a constant pressure is maintained inside the ducts and the pressure to be measured is applied to the outside of the fiber.
Because of the difference in thermal expansion coefficients between the two component portions of such a prior sensor, namely its core (the central portion), and its optical cladding (its peripheral portion), the sensor is sensitive to temperature.
In addition, the sensor is sensitive only to relative pressure.
Further, since the polarizers disposed at each end of the optical fiber are not subjected to pressure, the sensor is subject to metrological disturbances.
All of these difficulties reduce the advantages of such a sensor and the fields in which it can be used.
A pressure sensor has also been described, for example in German patent publication No 34 05 026, particularly for measuring differential pressure, which comprises a light transparent cylindrical body sensitive to the pressure of a fluid, liquid or gas, which is admitted in longitudinally extending cavities within this body. The cavities are closed at either end of the cylindrical body by sealing plates. These plates may be glued to the transparent body. These plates may also be made out of metal and clamped together against the ends of the cylindrical body by means of bolts. Each of these plates mounts sealing elements around the cavity openings in each end of the cylindrical body.
As in the case of the optical fiber sensor mentioned above, this sensor is sensitive only to relative pressure.
In addition, the junction between the sealing plates and the transparent sensor body gives rises to differential thermal dilatations which generate parasitic stresses in the sensor body. Furthermore this junction is also a cause for a lack of long term stability of the sensor. An object of the invention is to provide an optical sensor of absolute pressure and suitable for substantially mitigating or even completely eliminating the above-mentioned drawbacks and undesirable effects.